The name of staging/dev site matches to this list, they'll be automatic active:

TLDs that are considered as dev or staging:

•     *.dev
•     *.test
•     *.local
•     *.staging
•     *.example
•     *.invalid
•     *.myftpupload.com (GoDaddy)
•     *.cloudwaysapps.com (Cloudways)
•     *.ngrok.io (tunneling)

Subdomains that are considered as dev or staging:

•     local.*
•     dev.*
•     test.*
•     stage.*
•     staging.*
•     stagingN.* (SiteGround; N is an unsigned int)
•     *.staging.wpengine.com (WP Engine)
•     dev-*.pantheonsite.io (Pantheon)
•     test-*.pantheonsite.io (Pantheon)
•     staging-*.kinsta.com (Kinsta)

Additionally, if your domain is localhost (with any port), it will also be treated as a localhost domain.

Also, please note, that the activation is only necessary for getting new version updates. So you should use addons on staging websites without key activation, there is not any limit in functions and features if you don't activate it.

We do our best to make addons usage as easy as possible. Thus we don't limit any function and feature in addons even if it's not activated. This is the main reason why you can use it on DEV website without activation.

And as you know, the addon activation is only necessary for getting addon updates (the Single Site License is not a developer license. It's mostly designed to buy and activate on ready website). However on dev sites this is not a problem too. Addon updates are very rare, it maybe once per many months so I don't think this is a huge problem to do it manually. And yes we do search for a solution to solve this problem too, but I'm really sorry at the moment I we don't have that. So instead of purchasing 3 Site License I think it's better to do one manual update in 1-3 moth till we find some solution to make the dev sites activation more convenient for Single Site Licenses.

On the other hand

Also might I suggest that you update your warning text? 

"Please do not activate this plugin if you are on local host (127.0.0.1) or you use IP address to access to WordPress website. 
This plugin should only be activated if WordPress is located in WEB under a real domain name. "

I read that but I was on a real domain in the web when I activated my dev server, so I followed your instructions, but still failed to be able to update my plugins on the prod site without extra manual steps. It should specifically state that we should only activate it on our production web site. Or only activate it on the site where we will be doing automatic updates. 

Also every single site license should allow for dev / staging. Otherwise, your just encouraging people to steal it. We won't, but it's a nuisance.

Thank you for your interest in our add-ons and for contacting us.

In this case, just don't put the activation key in the "Activation Key" area.  

We've already mentioned it in the activation area:

Please do not activate this plugin if you are on local host (127.0.0.1) or you use IP address to access to WordPress website. 
This plugin should only be activated if WordPress is located in WEB under a real domain name. 

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If during the testing period some plugins will be updated, you can simply download the new version of the addon from your account page, then navigate to Dashboard > Plugins > Add new and upload the add-on ZIP file, then activate it.

Are we going to have to contact you every time we go from dev to prod with updates? If so please let me know now so I can get a refund and choose a different solution.